The present invention relates generally to cement backerboards and more specifically to wet formed mat applications for cement backerboards.
Interior and exterior construction boards with cores of plaster, cement, or hybrid materials, such as cement boards or gypsum boards, are used in a wide variety of indoor and outdoor structural applications. For example, cement boards are used as a support surface for overlying materials such as wood siding, stucco, aluminum, brick, tile, stone aggregate and marble. Also, cement boards are used in exterior insulating systems, commercial roof deck systems, masonry applications and exterior curtain walls.
Generally, cement boards contain a core formed of a cementitious material and low density fillers that are interposed between two facing layers. Facing materials advantageously contribute flexural and impact strength to the high compressive strength but brittle material forming the cementitious core. In addition, the facing material can provide a durable surface and/or other desirable properties to the cement board.
One material that has been used to form facing materials is alkaline resistant, binder coated glass fibers. Glass fiber facings provide increased dimensional stability in the presence of moisture and provide greater physical and mechanical properties to the cement board. These facing sheets are formed as randomly oriented fibrous glass mats or open mesh glass scrims formed from continuous glass yarns. Known methods for making cementitious boards consists of providing a continuous feed of facing material and depositing a cementitious slurry onto the top surface of the facing material. A second continuous feed of facing material is then applied to the top surface of the slurry. The slurry is dried to harden the cementitious composition and to integrate the facing material into the cement board. The cement board is subsequently cut to a predetermined length for shipping and eventual use.
Known glass scrim systems used to make the glass facing sheets typically include about 60% polyvinyl chloride (PVC) and glass scrim solutions that are manufactured in an expensive, two step process. Although some binder protection from highly alkaline conditions is required, the high binder levels are more likely due to process/coating related issues. Higher binder loading levels are required in existing glass facing sheets due to (i) throughput issues in a single strand coating operation, if precoated strands are employed; (ii) coating control issues that arise in coating pre-weaved fabrics; and (iii) possible wet out issues associated with thick, plasticized PVC resins. Biaxial 0/90 oriented scrims exhibit pitting in highly open porous regions.
It is therefore highly desirable to provide glass scrim system that can be formed in a single step operation that uses less binder material. It is also highly desirable that such a process will result in superior decorative finishes and better permeability control.
The present invention comprises combining an alkaline resistant binder with a permeable wet-formed mat composed of wet use chopped strands (WUCS), chopped dry strands or roving, and potentially unidirectional roving to create an open mat structure that can be used in cement backerboard applications.
A unique aspect of the present invention is the behavior of the wet chop strands and dry chop strands in the dispersion prior to forming the mat. Wet chop strands tend to spread out randomly within the whitewater dispersion. Dry chop strands will also randomly orient, however the material tends to stay together in the whitewater dispersion (like a log). Hence, the permeable matting formed is not a completely random network of wet and dry chop, thereby giving a mat with a higher degree of openness as measured by Frazier air permeability. This improves cement impregnation.
Other objects and advantages of the present invention will become apparent upon considering the following detailed description and appended claims, and upon reference to the accompanying drawings.